The present invention relates to a pressure sensor.
More particularly, it relates to a pressure sensor which has a housing, and a punch arranged between a diaphragm and a measuring element in the housing to introduce the pressure onto the measuring element.
The invention also deals with a process of producing such a pressure sensor. In the case of such a pressure sensor, known from German Offenlegungsschrift 31 25 640.6, the piezoresistive measuring elements, such as for example thick-film resistors of cermet, contactive plastic or metal, are applied to a base. The resistor element and the base are arranged as close as possible to the pressure chamber in order to be able to determine the prevailing pressure. Furthermore, the measuring signal is passed with the aid of electric leads to an electronic evaluation circuit arranged outside the housing of the pressure sensor. As a result, the piezoresistive elements and the electronic components have to be elaborately connected to one another with the aid of sheathed leads. Since the piezoresistive measuring element is exposed directly to the pressure, it is also exposed to the high temperatures prevailing in the combustion chamber. The flames spread there at a temperature of about 2000.degree. C., whereby stresses can occur in the housing. As a result, the pressure signal is falsified by the high temperatures.
Furthermore, European Preliminary Published Specification 85 111 895.0 discloses a pressure sensor in which the thick-film resistor is arranged on the bottom of a base. However, this pressure sensor is intended only for determining the pressure in distributor pumps. The high temperatures prevailing in the combustion chamber would falsify the measuring signal in the case of this design as well.
U.S. Pat. No. 4,645,965 describes a pressure sensor of which the measuring element consists of piezoelectric material. This piezoelectric element is arranged in a housing which has an opening towards the combustion chamber. This opening is closed off by a diaphragm, the piezoelectric element being in effective connection with this diaphragm, and consequently with the pressure prevailing in the combustion chamber, via a punch. The piezoelectric element bears against a shoulder formed in the housing. The use of the piezoelectric element also gives rise to constructional differences in comparison with the subject-matter of the application. In general, it is required that the measuring element operates potential-free. As a result, it is necessary that, in addition to the two contact discs already necessary for the piezoelectric element, a further insulating disc is fitted between the contact discs and the counterbearing. The contacting of the piezoelectric ceramic disc is possible only by elaborate welding of a wire or with the aid of a relief in the punch. If again a potential-free sensor is chosen, this effort would also double. Since the piezoceramic element has to be used as a discrete component, it is also relatively expensive in construction. Furthermore, several bonding points, elaborate and falsifying the measuring signal, are required. Even in a normal case with a piezoelectric disc and two contact discs, this results in four bonding points; with a potential-free sensor, it would be five bonding points. Owing to the measuring principle used as a basis, it is essential that the punch is bonded perpendicularly onto the piezoelectric element. Piezoceramic elements have a disadvantageous, relatively high temperature response of measuring sensitivity, which may lie between 15% and 70% in the application temperature range. Aging over the service life is typically 2% to 10%. The hysteresis of the piezoceramic of 5% to 25% is very high and thereby significantly restricts the accuracy of the combustion chamber pressure measurement. In particular, however, the multilayered construction of the piezoelectric arrangement makes relatively elaborate and complicated fitting into the housing of the pressure sensor necessary. In the case of piezoelectric elements, as distinct from piezoresistive elements, under the effect of pressure a charge is generated, and consequently a voltage, for producing the measuring signal. This voltage is picked up and evaluated. As distinct from this, in the case of piezoresistive elements a voltage is applied, and the electrical resistance in the piezoresistive element is changed by the pressure acting. Since, in the case of this pressure sensor, the piezoelectric element and the punch are fitted from one side into the bore and the latter is closed by a pot diaphragm, the position of the pot diaphragm and of the piezoelectric element have to be matched precisely to each other. This can be achieved only with a pot diaphragm.